Laser material processing is increasingly used for cutting, drilling, marking, and scribing a wide range of materials. Traditional mechanical processing produces rough surfaces and unwanted defects, such as micro cracks, which degrade and weaken the processed material. Laser material processing using a focused beam of pulsed laser-radiation produces more precise cuts and holes, having higher quality edges and walls, while minimizing the formation of unwanted defects. Progress in scientific research and manufacturing is leading to laser material processing of an increasing range of materials, while demanding higher processing speed and improved precision.
High-power laser-sources using solid-state gain-media produce fundamental laser-radiation having an infrared (IR) wavelength, typically a wavelength longer than about 750 nanometers (nm). IR laser-radiation is converted into visible and ultraviolet (UV) laser-radiation by harmonic generation in non-linear optical crystals. Short wavelength laser-radiation is capable of drilling smaller holes, making finer marks, and scribing finer features than longer wavelength radiation. UV laser-radiation is therefore preferred for processing many types of material. However, UV laser-radiation degrades optics, particularly optics that are also exposed to ambient oxygen and moisture. Harmonic generation crystals and any beam-shaping or beam-delivery optics are vulnerable to such damage.
Certain laser-sources produce beams of pulsed laser-radiation comprising pulses having femtosecond or picosecond pulse-duration, for example, pulses having a pulse-duration greater than about 100 femtoseconds (fs) and less than about 20 picoseconds (ps). Focused pulsed laser-radiation above a threshold intensity removes material from a workpiece by ablation, minimizing unwanted collateral damage caused by excess heating of surrounding material. Most materials have lower ablation thresholds at UV wavelengths than at IR fundamental wavelengths. Therefore, higher quality processing at higher speeds is possible using UV pulsed laser-radiation.
Many contemporary optoelectronic devices have composite structures. Light emitting diodes, photovoltaic cells, and touchscreens comprise a substrate overlaid with layers of different materials. The overlaying layers may include doped semiconductor layers, thin metal films, thin polymer films, and thin conductive-oxide films. Thin film layers are often deposited on the structure and then patterned by removing material. A focused beam of UV pulsed laser-radiation can selectively remove a thin-film without damaging underlying material, using the spatial selectivity provided by the short-wavelength laser-radiation combined with differences between the ablation thresholds of the layer materials.
Features are made in a material or patterned into a thin-film layer by moving the focused beam of laser-radiation in three dimensions through the material. Linear-translation stages support a workpiece and translate the workpiece in three dimensions through the focused beam up to a maximum controlled scan speed. Higher lateral scan speeds are accessible using state-of-the-art galvanometer-actuated motors to deflect the unfocused beam, translating the focused beam laterally through the workpiece.
There is need for a laser material-processing apparatus capable of generating a focused beam of UV pulses and precisely delivering the focused beam to a workpiece. The laser material-processing apparatus should be resistant to optical damage by the UV laser-radiation. Preferably, the pulses have sufficient energy to ablate a broad range of materials and the pulse-energy is controllable to selectively ablate thin films in composite structures.